This invention relates to a device for transporting sheets within a printing machine.
It is known in the art of sheet-fed printing machines to transport the sheets by means of grippers attached to gripper carriages, the gripper carriages being articulated on a pair of endless chains driven by chain wheels. The gripper carriages transport the sheets between the various components of the printing machine. A transport of this kind thus can take place between the individual printing units and, in particular, between the last printing unit and the sheet depository of a delivery stack. It is also known to transport the sheets from the last printing unit to a varnishing unit (with or without a verso-printing unit) and then from the varnishing unit to the sheet depository of the delivery stack. The chain wheels are mounted on a common continuous shaft which is driven from the sheet-guiding cylinder by means of a pair of gear wheels.
In the devices briefly described above, a transfer of the sheets between the gripper carriage and a sheet-guiding cylinder (for example, the impression cylinder of a printing unit or of a varnishing unit), takes place respectively in the region of the chain wheels. After the transfer of a sheet to a gripper carriage, a more or less pronounced curvature of the sheet takes place about the side which was last printed or varnished. Since the aim increasingly is first to print out the sheet completely--or provide it with a varnish coating--and only then transfer it to a gripper carriage, the sheet sometimes undergoes a very considerable looping around the take-off drum on which the chain wheels are mounted. A smearing of the freshly printed or varnished sheet can thus occur on parts of this drum.
The prior art discloses a plurality of feed and take-off drums which carry special devices on their outer circumferences in order to prevent smearing. By way of example, the sheet-guiding drum disclosed in German Patent Specification No. DE 3,930,747 C2 has a plurality of small round brushes arranged in close angularly spaced relation on its outer circumference over the width of the maximum sheet format. The sheet is said to be capable of being supported in a smear-free manner on the bristles of the round brushes. U.S. Pat. No. 3,054,348 discloses brush rings arranged on the drum axle instead of angularly spaced round brushes. Instead of brushes or brush rings, Great Britain Patent Specification No. 972,487 has proposed a number of supporting rollers which are arranged on bars and on which the sheet is likewise to be supported in a smear-free manner. The supporting rollers are adjustable by axial displacement to ink-free locations of the printing images. A particular disadvantage of this is that locations free of printing images are not always present and, in addition, a time-consuming adjustment of the supporting rollers is necessary.
A disadvantage of these previously known devices is that, in general, a smear-free sheet guidance is not always achieved in spite of the sometimes relatively high cost of construction.
The use of so-called air-cushion drums, which are known, for example, from German Patent Specification No. 1,561,043 and from German Patent Specification No. DE 3,827,071 C2, has in practice, afforded very good results with regard to smear-free sheet guidance. In small-format printing machines, however, these devices cannot be used for economic reasons in view of their comparatively high cost.